Synthetic antisense oligonucleotides have shown great promise as agents for the selective modulation of gene expression in tissue culture. Antisense oligodeoxynucleotides, modified oligodeoxynucleotides such as phosphoramidate, phosphorthioate, and methylphosphonate oligodeoxynucleotides as well as 2'-O-methyl phosphorothioate oligonucleotides have also shown substantial inhibition of human immunodeficiency virus (HIV). Although the mode of action of antisense oligomers may be complex, especially for phosphorothioates, charged deoxyoligonucleotides are generally thought to express their antiviral activity by specifically binding to mRNA or viral genomic RNA thereby promoting cleavage of the hybrid by a viral or cellular ribonuclease H. Duplexes of modified DNA (such as methyl phosphonates) with mRNA do not form substrates for ribonuclease H, and their antiviral activity probably results from translation inhibition.
Some of the limiting factors that affect the efficacy of previous oligonucleotides as potential antisense agents are: (i) lack of nuclease resistance, (ii) insufficient chemical stability, (iii) poor membrane permeability and therefore low rates of cellular absorption, (iv) low affinity for complementary RNA, (or double stranded DNA in the case of triple helix formation) and (v) toxicity/lack of selectivity. As such there exists a continuing need for new oligonucleotides and precursors thereto, and to processes for their preparation. The present invention addresses these needs.